Electric generation from wind is becoming an increasingly popular alternative to electric generation from other sources, for example, fossil fuels. More particularly, electrical power generated from the wind is expected to grow. A wind turbine assembly may be used to convert the wind to electrical power.
A typical wind turbine assembly, for example, the 2XL Wind Turbine, available from General Electric, Inc. of Fairfield, Conn., includes a support structure or pylon that extends skyward from the ground. A typical support structure may extend about five-hundred feet into the air from the ground. A housing including an electrical generator may be carried by an end of the support structure. A plurality of fan blades, for example, three, may extend outwardly from a shaft of the electrical generator. In operation, the force from incident wind currents causes the fan blades to rotate the generator shaft to cause electrical power to be generated. In some instances, up to 2.5 Megawatts may be generated.
Wind turbines may be available in a variety of configurations. For example, U.S. Pat. No. 6,790,007 to Gingras et al. discloses a vertical axis wind turbine including a rectangular shaped turbine extending from a pyramidal base. U.S. Patent Application Publication No. 2007/0013194 to Calley discloses a wind turbine attached to a monopole tower. U.S. Pat. No. 4,323,331 to Schachel et al. discloses a rotating tower that includes a plurality of structural beams. Other tower arrangements may include tubular towers, guyed towers, and lattice towers.
Geographic placement of a wind turbine may be problematic in areas near an airport or other radar site, for example. This is because a typical wind turbine assembly is not designed or constructed to address radar backscatter. More particularly, the support structure and electric generator housing may produce a static radar return, and the fan blades may produce a Doppler radar return. For example, a tip of a fan blade may reach a velocity of about 200 miles per hour, and thus, may cause an air traffic control radar to generate a “false alarm.” Additionally, the velocities of the fan blade toward and away from a weather radar, for example, may mimic a radar signature of a tornado vortex. The broad range of static and Doppler returns from a wind turbine or wind farm may reduce sensitivity to low and moderate altitude aircraft over a given azimuth sector.
Indeed, a wind turbine assembly may cause both reflectivity and motion interference that may impact any radar site that covers the region where it is located. A grouping of wind turbine assemblies, known as a wind farm, may further compound the interference. To reduce interaction with a radar installation it may be desirable to locate a wind farm beyond the radio line of site (due to the potential for refraction.) An estimate of the radar line of site may be made based upon the line of sight equation, described by Howard W. Sams in the ITT Reference Data for Radio Engineers, sixth ed., 1976, pages 12-28. By considering the distance to the radio horizon for the turbine and/or wind farm and that for the radar, an estimate of the line of site can be obtained.
According to the equation:d=(3 Kh/2)1/2=(3(1.33)(492/2))1/2=31.3 mileswhere h is the height of the antenna in feet, and K is the index of tropospheric refraction (usually taken to be 4/3 at sea level), the radio or radar horizon of the wind turbine will be approximately 31 miles. Using the same equation for a radar with a nominal height of 50 feet above the surface yields 6 miles. Thus over a smooth earth, 37 miles of separation may be desired to reduce a line of site interaction. Allowing an additional margin for signal refraction over the horizon, separations of 50 to 100 miles may be desired. This degree of separation may be relatively difficult to achieve, particularly in coastal areas. There may be the additional problem that oftentimes, wind farms are located on tall ridge lines to provide more efficient wind harvesting. In such cases they may be visible to a radar system hundreds of miles away. For these reasons, mitigation of the interaction between wind farms and radars by controlling the spatial separation of the two may not practical. Relying on geographical distance to control interaction may preclude at least a hundred of square miles of real estate from consideration for wind harvesting. By unnecessarily eliminating large tracts of land from consideration, many landowners will be excluded from participation in wind energy generation. Additionally, directional broadcast stations such as those in the range of 530 to 1710 kHz may employ directional antenna arrays whose radiation pattern nulls may be altered or degraded by proximity to nearby wind turbine structures.
U.S. Patent Application Publication No. 2009/0202347 to Rugger discloses a control system for mitigating effects of a wind turbine on a radar system. The control system includes a sensor configured to detect an operating condition of the radar system. The control system also includes a processor configured to receive an operating condition of the wind turbine and determine a rotation modification sequence based on the operating condition of the radar system and the operating condition of the wind turbine. The control system further includes a controller configured to apply the rotation modification sequence to the wind turbine. In other words, the control system manipulates the wind turbine to mitigate the effects on the radar system.
U.S. Patent Application Publication No. 2009/0121491 to Mikkelsen discloses a wind turbine that includes a conductive film layer on an enclosure structure. The conductive film layer forms a shield enclosing the part or parts and protects against electromagnetic fields. The conductive film layer may be a radar neutral material. The radar neutral material may behave so that radar located in the vicinity of the wind turbine and radiating radar-RF energy in the direction of the wind turbine will receive a degraded amount or substantially no radar-RF energy reflected from the material at its receiver. However, this approach may not work well for a wind turbine, as it may increase the mass of the blade.
U.S. Patent Application Publication No. 2006/0169930 to Butler discloses an apparatus for reducing the electromagnetic radiation reflected from structures in the direction of electromagnetic radiation receiving equipment is disclosed. The apparatus is situated between an object and a source of electromagnetic radiation. The apparatus includes an array of at least one substantially reflective panel arranged such that the array reflects and disperses incident electromagnetic radiation away from the receiving equipment.
Another approach provides a radar absorbing blade. The radar absorbing blade may act like a Salisbury screen so that incoming radar waves bounce off two surfaces of the blade that are precisely spaced so that reflections interfere and cancel each other out. Such an approach has been proposed by Vestas Wind Systems of Randers, Denmark.
Another approach, proposed by the QinetiQ Company of Farnborough, Hampshire, modifies an inside portion of each blade with layers of circuits and reflectors that would reduce the strength of a radar return from the blades.
Other references disclose the use of different materials in a wind turbine, for example, U.S. Pat. No. 6,224,341 to Fricke discloses a fan blade that is selectively filled with a damping fill material. The damping fill material may include a ceramic material, an ore or refractory material, and glass micro-bubbles, and may depend on a given application, for example, a gas turbine blade.
U.S. Patent Application Publication No. 2004/0253114 to Gunneslov et al. discloses a wind turbine blade that includes pre-fabricated strips arranged in a sequence along an outer periphery of the wind turbine blade. The pre-fabricated strips may include carbon fibers, glass fibers, wood strips, or composite strips formed as hollow tubes.